https://doi.org/10.5194/acp-2019-669 Preprint. Discussion started: 10 September 2019 c Author(s) 2019. CC BY 4.0 License. Deposition, recycling and archival of nitrate stable isotopes between the air-snow interface: comparison between Dronning Maud Land and Dome C, Antarctica V. Holly L. Winton1, Alison Ming1, Nicolas Caillon2, Lisa Hauge1, Anna E. Jones1, Joel Savarino2, Xin 5 Yang1, Markus M. Frey1 1British Antarctic Survey, Cambridge, CB3 0ET, UK 2University of Grenoble Alpes, CNRS, IRD, Grenoble INP, IGE, F-38000 Grenoble, France Correspondence to: V. Holly L Winton ([email protected]) Abstract - 15 - 10 The nitrate (NO3 ) isotopic composition δ N-NO3 of polar ice cores has the potential to provide constraints on past ultraviolet (UV) radiation and thereby total column ozone (TCO), in addition to the oxidising capacity of the ancient atmosphere. However, understanding the transfer of reactive nitrogen at the air-snow interface in Polar Regions is paramount for the 15 - - - interpretation of ice core records of δ N-NO3 and NO3 mass concentrations. As NO3 undergoes a number of post-depositional 15 - processes before it is archived in ice cores, site-specific observations of δ N-NO3 and air-snow transfer modelling are 15 necessary in order to understand and quantify the complex photochemical processes at play. As part of the Isotopic Constraints - 15 on Past Ozone Layer Thickness in Polar Ice (ISOL-ICE) project, we report new measurements of NO3 concentration and δ N- - NO3 in the atmosphere, skin layer (operationally defined as the top 5 mm of the snow pack), and snow pit depth profiles at Kohnen Station, Dronning Maud Land (DML), Antarctica. We compare the results to previous studies and new data, presented here, from Dome C, East Antarctic Plateau. Additionally, we apply the conceptual one-dimensional model of TRansfer of 20 Atmospheric Nitrate Stable Isotopes To the Snow (TRANSITS) to assess the impact of photochemical processes that drive the 15 - - - archival of δ N-NO3 and NO3 in the snow pack. We find clear evidence of NO3 photolysis at DML, and confirmation of our - hypothesis that UV-photolysis is driving NO3 recycling at DML. Firstly, strong denitrification of the snow pack is observed 15 - through the δ N-NO3 signature which evolves from the enriched snow pack (-3 to 100 ‰), to the skin layer (-20 to 3 ‰), to - the depleted atmosphere (-50 to -20 ‰) corresponding to mass loss of NO3 from the snow pack. Secondly, constrained by - 25 field measurements of snow accumulation rate, light attenuation (e-folding depth) and atmospheric NO3 mass concentrations, 15 - - the TRANSITS model is able to reproduce our δ N-NO3 observations in depth profiles. We find that NO3 is recycled three times before it is archived (i.e., below the photic zone) in the snow pack below 15 cm and within 0.75 years. Archived δ15N- - - -1 - NO3 and NO3 concentration values are 50 ‰ and 60 ng g at DML. NO3 photolysis is weaker at DML than at Dome C, due 15 - primarily to the higher DML snow accumulation rate; this results in a more depleted δ N-NO3 signature at DML than at Dome 30 C. Even at a relatively low snow accumulation rate of 6 cm yr-1 (water equivalent; w.e.), the accumulation rate at DML is great 1 https://doi.org/10.5194/acp-2019-669 Preprint. Discussion started: 10 September 2019 c Author(s) 2019. CC BY 4.0 License. - 15 - enough to preserve the seasonal cycle of NO3 concentration and δ N-NO3 , in contrast to Dome C where the profiles are 15 - smoothed due to stronger photochemistry. TRANSITS sensitivity analysis of δ N-NO3 at DML highlights that the dominant 15 - factors controlling the archived δ N-NO3 signature are the snow accumulation rate and e-folding depth, with a smaller role from changes in the snowfall timing and TOC. Here we set the framework for the interpretation of a 1000-year ice core record 15 - 15 - 35 of δ N-NO3 from DML. Ice core δ N-NO3 records at DML will be less sensitive to changes in UV than at Dome C, however 15 - the higher snow accumulation rate and more accurate dating at DML allows for higher resolution δ N-NO3 records. 1 Introduction - Nitrate (NO3 ) is a naturally occurring ion formed by the oxidation of nitrogen, and plays a major role in the global nitrogen cycle. It is one of the most abundant ions in Antarctic snow and is commonly measured in ice cores (e.g. Wolff, 1995). Nitrate - 40 in polar ice provides constraints on past solar activity (Traversi et al., 2012), NO3 sources and the oxidative capacity of the atmosphere (Geng et al., 2017;Mulvaney and Wolff, 1993;Hastings et al., 2009;Hastings et al., 2004;McCabe et al., - 2007;Savarino et al., 2007;Morin et al., 2008). However, NO3 is a non-conservative ion in snow, and due to post-depositional - processes (e.g. Mulvaney et al., 1998;Zatko et al., 2016), the interpretation of NO3 concentration records from ice core records - 15 is challenging (Erbland et al., 2015). The recent development of the analysis of nitrogen isotopic composition of NO3 (δ N- - - 45 NO3 ) in snow, ice and aerosol provides a powerful means to understand the sources and processes involved in NO3 post- - depositional processes, i.e., NO3 recycling at the interface between air and snow. Primary sources of reactive nitrogen species to the Antarctic lower atmosphere and snow pack include the sedimentation of polar stratospheric clouds (PSC) in late winter (Savarino et al., 2007) and to a minor extent advection of oceanic methyl nitrate (CH3NO3) and peroxyacyl nitrates (PAN) (Jacobi et al., 2000;Jones et al., 1999;Beyersdorf et al., 2010). In the stratosphere, - 50 NO3 is produced through the stratospheric oxidation of nitrous oxide (N2O) from extra-terrestrial fluxes of energetic particles and solar radiation, whereas in the troposphere lightning and biomass burning provide background tropospheric reactive nitrogen species to the snow pack (Savarino et al., 2007;Wolff, 1995;Wagenbach et al., 1998). A local secondary source of reactive nitrogen (nitrous acid (HONO), nitrogen oxides (NOx)) originates from post-depositional processes driven by sunlight leading to re-emission from the snow pack and subsequent deposition (Savarino et al., 2007;Frey et al., 2009). - 55 Local nitrogen dioxide (NO2) emissions in Polar Regions are produced from NO3 photolysis in the snow pack under sunlit conditions (Jones et al., 2001;Honrath et al., 1999;Oncley et al., 2004). Nitrate photolysis occurs at wavelengths (λ) = 290-345 - nm with a maximum at 320 nm. Photolysis rate J depends on the adsorption cross section of NO3 , the quantum yield and - actinic flux within the snow pack. Photochemical production of NO2 is dependent on the NO3 concentration in the snow pack, the snow pack properties, and the intensity of solar radiation within the snow pack. The latter is sensitive to solar zenith angle 60 and snow optical properties i.e. scattering and adsorption coefficients, which depends on snow density and morphology, and the light absorbing impurity content (France et al., 2011;Erbland et al., 2015;Zatko et al., 2013). Recently, Zatko et al. (2016) found that the range of modelled NOx fluxes from the snow pack to the overlaying air are similar in both Polar Regions due to 2 https://doi.org/10.5194/acp-2019-669 Preprint. Discussion started: 10 September 2019 c Author(s) 2019. CC BY 4.0 License. - the opposing effects of higher concentrations of both photolabile NO3 and light absorbing impurities (e.g. dust and black carbon) in Antarctica and Greenland respectively. At Concordia Station on Dome C in East Antarctica, the light penetration 65 depth (e-folding depth) is ~10 cm for wind pack layers and ~20 cm for hoar layers (France et al., 2011). Based on the propagation of light into the snow pack, the snow pack can be divided into three layers. The first layer is known as the skin layer (a few mm thick) where direct solar radiation is converted into diffuse radiation. The second layer is called the active photic zone (below the skin layer layer), where solar radiation is effectively diffuse and the intensity of the radiation decays exponentially (Warren, 1982). The third layer is called the archived zone (below the active photic zone), where no 70 photochemistry occurs. Previous research has focused predominantly on the high elevation polar plateau (Dome C). Here, the exponential decay of - - NO3 mass concentrations in the snow pack and thus post-depositional processing of NO3 were attributed to either evaporation or ultra-violet (UV)-photolysis (Röthlisberger et al., 2000;Röthlisberger et al., 2002). The open debate of which post- - depositional process controlled NO3 mass concentrations in the snow pack led to the use of a new isotopic tool, the stable 15 - 75 isotopic composition of nitrate (δ N-NO3 ) (Blunier et al., 2005). More recently, theoretical (Frey et al., 2009), laboratory (Meusinger et al., 2014;Erbland et al., 2013;Erbland et al., 2015;Shi et al., 2019;Berhanu et al., 2014), and field (Erbland et - al., 2013;Frey et al., 2009) evidence show that NO3 mass loss from the surface snow to the overlying atmosphere and its - associated isotopic fractionation is driven by photolysis. The physical release or evaporation of NO3 is negligible (Erbland et al., 2013;Shi et al., 2019). - 80 At Dome C, the large redistribution and net mass loss of NO3 below the skin layer and the simultaneous isotopic fractionation 15 - - of δ N-NO3 in the snow pack indicate that post-depositional processes significantly modify the original NO3 concentration 15 - 15 - and δ N-NO3 composition (Frey et al., 2009).